In the flue gases generated by the large facilities which burn coal or fuel oil, in particular power generation plants, the concentration of SO.sub.2 is relatively low, but, on considering the enormous gas volumes discharged to the atmosphere, the amounts of SO.sub.2, on an absolute basis, reach environmentally unacceptable levels. Installing sulfur removing units in these facilities has been a need for years, and is now also imposed by law.
The flue gases from sulfur containing metal ore roasting facilities are presently sent to H.sub.2 SO.sub.4 productions, but owing to the present difficulty met in selling the latter, an alternative process which converts SO.sub.2 from exhaust gases into elemental sulfur rather than into H.sub.2 SO.sub.4 could contribute to solve the marketing problems.
The solutions offered by the prior art for flue gases desulfurization are substantially two:
the limestone/chalk processes which convert SO.sub.2 into a solid residue comprised of calcium sulfate/sulfite which must be disposed of in specially designed areas, PA1 the regenerative processes which convert SO.sub.2 into marketable sulfur forms, and do not generate residues to be disposed off. These processes are the only ones which are completely acceptable from a strict environmental protection view point, but involve decidedly higher investment and operating costs, even if these operating costs may be partially compensated for by the income deriving from sales of sulfur, liquid and/or H.sub.2 SO.sub.4, produced by the same processes. PA1 transformation into sulfur by the Claus process; PA1 liquefaction to produce liquid SO.sub.2 ; PA1 conversion into H.sub.2 SO.sub.4 by the contact method. PA1 the method taught by U.S. Pat. No. 3,475,122, using 3-compartment electrolysis cells for producing NaOH for SO.sub.2 absorption and acid for neutralizing the basic solution saturated with SO.sub.2, with SO.sub.2 being consequently released; and PA1 the process taught by U.S. Pat. No. 4,107,015, in which the SO.sub.2 -saturated alkaline solution is charged to an electrodialysis membrane separation unit; from the acidic compartments a solution is obtained which is suitable for SO.sub.2 stripping, and from the basic compartments a solution is obtained which contains the necessary NaOH for absorbing further amounts of SO.sub.2. PA1 directly produces elemental S, a substance which is in great demand by the market, and is easy to be stored; PA1 reduces the number of steps and of the necessary equipment units; PA1 is easily operated and controlled, with low operating costs; PA1 does not produce any substances which may require being further processed in order to be rendered useable, and
At present, on a practical basis the processes of the first type, in which removal of SO.sub.2 is based on the reaction of the latter with a suspension of lime/chalk, are more widely diffused. The exhaust gases from the boilers are caused to flow through a wash tower in which they are contacted, in countercurrent, with a slurry containing lime/limestone, kept continuously recycled and subdivided into extremely fine droplets in order to favor the intimate contact thereof with the gas and, consequently, SO.sub.2 absorption.
Calcium sulfite and sulfate are formed, which are usually converted into a homogeneous product by means of an end oxidation.
The yields of SO.sub.2 extraction may reach 90%.
The reactant consumptions are always largely higher than the stoichiometric amounts, owing to the low solubility of lime and chemical inertness of limestone.
Among this type of processes, in particular the Babcock-Hitachi, Combustion Engineering, Costain Deutsche Babcock and Lodge Cottrel processes have been successfully adopted.
An analogous process is the double-alkali process, which is based on SO.sub.2 absorption by a sodium carbonate solution.
The extraction yield is good (about 90%), the reactant consumption practically corresponds to the stoichiometric requirements, the process management is relatively simple, but for partial alkali regenerations the treatment with lime is used, so also in this case calcium sulfite and calcium sulfate are formed which, together with the excess of lime, must be disposed off to landfill.
Among the regeneration processes, the Wellman-Lord process, using the sulfite-bisulfite system, gained a leading position. Due to the effect of absorbed SO.sub.2, the absorbent sodium sulfite solution is converted into bisulfite and can be thermally regenerated; sodium sulfite is formed again and an SO.sub.2 stream is released which has a high enough concentration for the subsequent alternative treatments:
The absorption step is carried out in an absorption tower, with a removal yield which usually is higher than 90%. The sulfite regeneration is carried out inside a forced-circulation evaporator.
During the absorption step, a portion of sulfite is converted into sulfate, so from this process also, this product is obtained, together with SO.sub.2.
The investment cost is high, facility management is complex.
Also the process based on use of Mg oxide developed by Philadelphia Electric/UE&C is a wet-type regenerative process; for the absorption, a suspension of MgO in water is used; Mg sulfite is formed which can be decomposed by calcination into MgO and SO.sub.2 (also MgSO.sub.4, which is also partially formed during the absorption step, can be reduced to MgO and SO.sub.2 if the calcination is carried out in the presence of coal).
Therefore, the only product obtained is a concentrated SO.sub.2 stream, which can be submitted to the usual treatments yielding commercial grades of this product.
The investment cost is high and the process management is quite complex.
The citrate process uses, for desulfurization, a solution of citric acid and sodium citrate. This solution is an extremely good absorbant for SO.sub.2, because it acts as a pH buffering agent. The wash solution enriched with SO.sub.2 is sent to a reactor in which it is reacted with H.sub.2 S (obtained by recycling a portion of produced sulfur and causing it to react with fuel gas in an adjacent facility), so as to form a slurry containing 10% of elemental S.
The high-purity sulfur is separated from the solution which is sent back to the absorption step. The presence of sodium sulfate which is formed by oxidation during the absorption makes it for a purge to be carried out in order to remove it.
Therefore, the process products are sulfur and sodium sulfate.
The high investment cost and the complexity of process management have limited the diffusion thereof.
For the sake of completeness, the following should be cited as well:
All the above described regenerative processes aim at obtaining a concentrated enough SO.sub.2 stream to be suitable for being converted, by means of a downstream facility, into a marketable product (liquid SO.sub.2, H.sub.2 SO.sub.4 or elemental sulfur).